1. Technical Field
The present invention relates to an imaging device and a focus control method.
2. Related Art
Recently, as the resolution of a solid-state imaging element such as a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor becomes higher, a demand for an information device having an imaging function such as a digital still camera, a digital video camera, a cellular phone such as a smart phone, and a personal digital assistant (PDA) is rapidly increasing. In the meantime, the information device having an imaging function as described above is referred to as an imaging device.
In such an imaging device, as a focus control method which focuses on a major subject, a contrast auto focus (AF) method or a phase difference AF method is employed. Since the phase difference AF method may detect a focusing position with high precision at a high speed as compared with the contrast AF method, the phase difference AF method is widely employed in various imaging devices (see, e.g., Patent Literature) (JP-A-2007-219539), Patent Literature 2 (JP-A-7-143391) and Patent Literature 3 (JP-A-2009-092824)).
According to the phase difference AF method, outputs of a pair of phase difference detecting sensor rows are obtained as data and correlation of the outputs of the pair of sensor rows is obtained. Specifically, data of one of the sensor rows is assumed as A[1] . . . A[k] and data of the other sensor row is assumed as B[1] . . . B[k] and a value of “d” when an area S[d] enclosed by two data waveforms calculated by the following equation is minimized when the two data are displaced by “d” is calculated as a phase difference amount, and a focus lens is driven based on the phase difference amount.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            S            ⁡                          [              d              ]                                =                                    ∑                              n                =                1                            k                        ⁢                                          (                                                      A                    ⁡                                          [                                              n                        +                        d                                            ]                                                        -                                      B                    ⁡                                          [                      n                      ]                                                                      )                            2                                      ⁢                                  ⁢                              d            =                          -              L                                ,          …          ⁢                                          ,                      -            2                    ,                      -            1                    ,          0          ,          1          ,          2          ,          …          ⁢                                          ,          L                                    (        1        )            
Patent Literature 1 discloses an imaging device which performs an optimal focus operation on a photosensitive member by changing a shift pitch (corresponding to an increase/decrease amount of d) of two images when correlation of two images formed on the pair of phase difference detecting sensors is operated in accordance with information (information on a permissible diameter of a circle of confusion) on the photosensitive member (a film or an imaging element) of a camera main body.
Patent Literature 2 discloses an imaging device which calculates a phase difference amount by setting L in Equation 1 as a first value, moves the focus lens based on the phase difference amount, and then recalculates the phase difference amount by decreasing L to be smaller than the first value, and moves the focus lens based on the phase difference amount.
Patent Literature 3 discloses an imaging device which varies a width of the phase difference detecting sensor row which is used to calculate the phase difference amount in accordance with a zoom magnification.